The present invention is related to an apparatus and its method for mixing video signals for simultaneous presentation on a video monitor device, and more particularly relates to an apparatus and its method for mixing inversely proportional portions of a video signal in television format with a computer generated graphics signal such that the voltage of the combined signal is below a defined limit.
With the advent of personal computers which generate graphics signals compatible with television signals, it has become desirable to combine computer generated graphics signals, to include alphanumeric characters, with video signals from video sources such as video disc recorders, video tape recorders, television station broadcast signals, television cameras, and other sources of video signals in a television format.
Systems are known in which alphanumeric characters, such as captioned TV characters, are embedded in certain portions of the TV signal, such as during blanking, and then decoded and displayed on a TV screen as desired.
Other systems are known in which alphanumeric characters, such as a time-of-day display, channel numbers or other caption data, are generated by character generators and added to a TV signal to be displayed on a TV screen along with a normally transmitted television video signal.
U.S. Pat. No. 4,425,581 issued Jan. 10, 1984 to Schweppe et al. discloses a system for overlaying a computer generated video signal on an NTSC video signal. The Schweppe et al. system. includes a key switch circuit which receives an NTSC video signal and a variable DC level signal. The key switch circuit then provides for display, either the NTSC video signal or the variable DC level signal depending upon the presence of a stripped computer video signal and an inverted stripped computer video signal.
Other systems are known in which the characters to be displayed are written during vertical retrace of the video signal, or in which a set voltage replaces the video signal within the outline of the characters to be shown on the TV screen, or in which the character signal is added directly to the video signal.
Many of these systems result in a display in which the computer generated signal replaces the video signal thereby obscuring that portion of the overlaid video signal with the overlaying computer generated signal. In those cases where the two signals are merely added together, one or both signals are obscured when the added signals at the point of overlap exceed a maximum level such that at the point of overlap, a flair or white out is produced.
Other patents of interest are:
U.S. Pat. No. 3,499,979 issued Mar. 10, 1970; PA0 U.S. Pat. No. 3,584,142 issued June 8, 1971; PA0 U.S. Pat. No. 3,731,282 issued May 1, 1973; PA0 U.S. Pat. No. 3,891,792 issued June 24, 1975; PA0 U.S. Pat. No. 3,898,644 issued Aug. 5, 1975; PA0 U.S. Pat. No. 4,040,088 issued Aug. 2, 1977; PA0 U.S. Pat. No. 4,051,532 issued Sept. 27, 1977; PA0 U.S. Pat. No. 4,117,511 issued Sept. 26, 1978; PA0 U.S. Pat. No. 4,305,131 issued Dec. 8, 1981; PA0 U.S. Pat. No. 4,396,944 issued Aug. 2, 1983; PA0 and U.S. Pat. No. 4,475,164 issued Oct. 2, 1984.